As intermittent renewable energy sources such as wind and solar increase their share of overall energy production, a method is required to compensate for their intermittency and to match the demand of a power grid in real time. Numerous methods have been discussed to stabilize intermittent renewables, including grid extension to average over larger sets of intermittent assets, demand-side management, ramping of conventional assets, and energy storage, including technologies such as electrochemical storage, such as Li-ion, Na/S, and Na/NiCl2, thermal storage, power to gas, and other energy storage technologies. Flow batteries are one particularly promising technology used to store electrical energy and stabilize power flow from intermittent renewable energy sources. While the most prominent flow battery couple uses vanadium at different oxidation states at each electrode, there are many other couples under consideration, with reactants in the gas, liquid, and solid forms.
One particular flow battery reacts H2 and Br2 to form HBr on discharge. One advantage of this couple is that the H2 reaction is kinetically rapid when catalyzed and the Br2 reaction is kinetically rapid, whether or not the reaction is catalyzed. Rapid kinetics and the ability to obtain components from the related system reacting H2 and O2 in a proton-exchange membrane fuel cell enable the H2/Br2 chemistry to achieve a very high power density. This high power density reduces the area required for a given amount of power and, since the system cost has a significant dependence on the total area over which the reactions are carried out, holds promise for energy storage with reduced costs.
FIG. 1 illustrates a schematic diagram of a conventional H2/Br2 flow battery cell 100 including a number of cell layers included in the cell 100. FIG. 1 illustrates the reactions occurring during battery discharge, though reversing the illustrated reactions results in charging the battery cell 100. Hydrogen gas (H2) is sent through a hydrogen gas channel 104 into a negative electrode 108, at which a porous medium 112 and a catalyst layer 116 are present. The catalyst layer 116 is typically made of Platinum (Pt) to catalyze H2 oxidation on discharge and hydrogen ion (H+) reduction on charge. During discharge, H+ is produced from the H2 gas via the reaction H2→2H++2e−. The H+ passes through a membrane 120 to a positive electrode 124, where it is combined with Br− to form hydrobromic acid (HBr) via the reaction Br2+2H++2e−→2HBr. The membrane 120, in one or more embodiments, is an ion-exchange membrane, such as the cation-exchange membrane Nafion, or a separator with pores through which the H+ passes. On the positive electrode side, a liquid solution composed of Br2 and HBr is delivered and flows through a Br2/HBr channel 128 by the positive electrode 124. A porous electrode 124 is always present, though the catalyst 132 layer is optional on the positive electrode side since the kinetics of the Br2 reaction (Br2+2e−→2Br− on discharge and 2Br−→Br2+2e− on charge) are fast even on uncatalyzed carbon. The presence of HBr, which typically dissociates to form H+ and Br−, allows for the conduction of ionic current within the porous electrode. The electrons are passed through an external circuit, where useful work may be extracted (discharge) or added (charge) to the circuit.
In many battery systems, reactants exist in a solid form, and only the shuttle ion is mobile. However, in flow batteries, at least one of the reactant materials exists in a liquid or a gas phase. Many of these liquid or gas reactant materials have a tendency to cross through the cell membrane. When these crossover events occur, they can negatively affect the performance of a flow battery. For example, the mobile reactant that has crossed over can directly chemically react with other reactants, which causes a loss of energy since the reaction does not take place electrochemically, and therefore does not generate any useful work. Additionally, crossover events can reduce the charge and discharge capacity of a battery since the reactant material that has crossed over is no longer capable of participating in electrochemical charge and discharge cycles.
Ion crossover results in various problems for different flow battery couples. For example, in a Zn/Br2 battery couple, Br2 in the liquid form can cross over to the zinc metal side, reacting directly on the zinc side instead of at the positive electrode where useful work is produced. In a vanadium/vanadium flow battery couple, vanadium at different oxidation states can cross through the separator and react chemically, reducing the energy efficiency, for example V5++V3+2 V4+. In an iron/chromium battery couple, crossover through the separator causes reactions that reduce the energy efficiency and may result in the need to separate the crossed over ions.
In an H2/Br2 flow battery system, several materials have a tendency to migrate through the cell membrane. For example, H2O, Br2, and HBr (as well as polybromide species such as HBr3) molecules may migrate from the liquid side of the cell to the gaseous side of the cell, while H2 molecules may migrate from the gaseous side of the cell to the liquid side of the cell. The rates of these different crossover processes are a function of material properties, material concentrations, cell conditions, and whether the cell is being charged or discharged.
Consequently, what is needed is a flow battery system that reduces reactant crossover, which generally results in a loss of energy efficiency and battery capacity. In addition, what is needed is a system and method to overcome the problem of reactant mixing in the case of hydrogen/bromine chemistries, with a particular focus on the return of bromine species to the bromine side of the cell.